PHYSICALREVIEW 8VOLUME46,NUMBER1515OCTOBER1992-ISoft-x-ray-emissionstudies ofbulkFe3Si,FeSi,andFeSiz,andimplantediron silicidesJ. J.Jia,T.A.Callcott,W.L.O'Brien,andQ.Y.DongUniversityofTennessee, Knoxviile,Tennessee37996D.R. MuellerandD. L. EdererRational InstituteofStandards andTechnology,Gaithersburg,Maryland20899Z. TanandJ.I.BudnickUniversityofConnecticut,Storrs,Connecticut 06269(Received 27April1992)Bulk ironsilicidesandimplantediron silicideshave been studiedbysoft-x-rayemission(SXE)spec-troscopy.The SiL23emissionspectraof thesematerials are measured. For bulksilicides,thesespectraprovideameasure ofs-andd-type partial densityof states(PDOS) localized on the Si sites. Wecomparethemwithavailable band-structurecalculationsandalso withphotoemission measurements.Forim-plantedsystems,the SiL»emissionspectra provideuseful informationaboutthesilicideformationpro-cess withthe variationofimplantdoses.I.INTRODUCTIONInagreatnumber ofstudies'conductedon thetransition-metal(TM)silicides andmetal-siliconinter-faces,iron silicides have receivedfarless attention thancobalt andnickel silicides.Onlyasmall number ofelec-tronic structure studieshave beenperformedon ironsili-cides.Partly,this is duetothe lowertechnologicalpo-tential of iron silicides atpresent comparedwitheitherCoor Nisilicides,andpartlyit is due to thecomplexcrystalstructure of these materials. Yetiron silicideshaveveryinterestingelectronic andmagnetic properties.A morecomprehensive understandingoftheir electronicstructurescertainlywillhelptoexploretheseproperties.Oneof the threepurephasebulk Fesilicides,FeSi2isagoodsemiconductor andshowspotentialuseincombin-ingsilicon-baseddigital technologywith new optoelect-ronic devices.Another,Fe3Si,has drawn considerableinterestdue to thesiteoccupationpreferenceofsubstitu-tionaltransition-metalimpuritiesin thismaterial. But ithasbeen studiedmostlybynuclearmagneticresonance(NMR)and neutronscatteringtechniques.The magnet-icpropertiesandstructural stabilityof thethird, FeSi,arealsoveryinteresting.A clearpictureof itselectronicstructureandchemicalbondingisneeded tofullyunder-stand theseproperties.However,ourknowledgeofthesematerialsis stillincompleteasfar astheelectronicstruc-ture andchemicalbondingareconcerned.We havemadesoft-x-ray-emission(SXE)measurementsonthesematerialsin order togainmore experimentalinformationon thefilled bandsofthese compounds.SXEspectrosco-pyisaprovenmethod ininvestigatingthe bulkelectronicstructuresof solidmaterials.Ourpreviousstudies'havedemonstratedtheusefulnessofSXEspectroscopyinstudyingbulk silicidesandimplantedsilicides.Inpartic-ular,thes-andd-likepartialdensityofstatesofthevalence electronslocalized onSi sitescan be effectivelystudiedbythis method.Inthispaper,wewillpresentthe SiL23emission spec-tra of these bulk Fe silicides and also of someiron-implantedsiliconsamples.In SXEspectroscopy,ener-geticelectrons orphotonsareused togeneratevacanciesinthe core levels ofatoms withinthespecimen.Soft xraysareemittedas valenceelectronsradiativelyrecom-bine with thecore-levelvacancies. Theenergydistribu-tionofthesephotonsare detected andanalyzed,whichprovidesinformationaboutthe filled valence orconduc-tion bandsofthematerialsstudied.Because thespectraaregeneratedinradiative transitionsto alocalized corehole,thespectroscopyischemicallyselective and pro-vides a localdensityof states(LDOS)for each element ofacomplexsolid. Inaddition,the radiative transitionsobeythedipoleselection rule forangularmomentum sothat SXEspectra formallyrepresentangular-momentum-selectedpartial densityofstates(PDOS).Also,formostoperatingconditions,bothexcitation andescapedistancesaresufficientlygreatso thatSXE spec-troscopyis a bulkprobewhichisrelativelyinsensitivetosurface conditions.Bymeasuringthe SiL23emissionspectraofironsili-cides,thelocal s anddLPDOS'sfor the Si sites can beeffectivelyprobedbythismethod. Thisinformationisnot availablefromphotoemissionmeasurements duetothedominance of the Fe delectrons in photoemissionspectra.We will discuss theinvolvement ofsilicon selec-tronsinbondingandcompareourresultswithavailablecalculations and photoemissionresults.Generalagree-ment is found betweenourspectrumonFeSi2andtheoretical calculations aswellaspreviousexperimentalresults.Somecomparisonscan alsobemadewithlessde-tailedcalculationsavailable forFe3Si.Nodetailedband-structure calculationswithprojectedLPDOSexist intheliteraturefor FeSi. However,somecleartrendscan beidentifiedingoingfrom iron-poortoiron-richcom-4694461992TheAmericanPhysicalSociety46SOFT-X-RAY-EMISSIONSTUDIES OFBULKFe3Si,FeSi,...poundsthatgiveinformationabout the evolutionofbondingin these materials.II. EXPERIMENTl 1 I(IIIl(I I IIfI Il I(II I I(Ii yy(pggFe SiThebulk FeSiandFeSi2 sampleswerepreparedbycompressingcommercially obtainedpowdersintopellets.Thepowdershavecrystallitesvisiblylargeenoughtorepresentthebulkproperties.TheFe3Sisampleisasin-glecrystalpreparedbyarcmelting.Itscompositionandcrystalstructure werecheckedbyx-rayscattering.Theiron-implantedsampleswerepreparedbyuniformlyim-plantingpolishedsilicon(100)wafers.Ascanningbeamof150-keVFe+ionswas used forimplantationwithsub-stratetemperature maintainedat about350'Cin avacu-um of 10 Torr.Thesampleholder was surroundedbya liquid-nitrogentrapandwarmedbya heaterwithtem-peraturesmonitored and stabilizedduringthe implanta-tion.Thesampleswestudied here haveimplantationdosesof 5 X10',7 X10',and 1X10'Fe/cm .SXEmeasurementswere carried out on our high-efficiencySXEspectrometerinstalledat the NationalSynchrotronLightSource,Brookhaven NationalLabora-tory.Spectrometerand detectorhave been describedelsewhere.''"Our measurements were conductedwith a600-lines/mmgratinganda100-p,minputslit.Instru-mental resolution isestimatedtobebetter than0.2eVataphotonenergyof 100 eV. Measurements were madeinan UHVchamber at apressureof 5X10Torr.Emis-sionwasexcitedbya100-pA,3-keVelectron beamfo-cusedto a1-mmspot.Thesampleswereplacedonaholderlocated3or 4mmawayfrom the entrance slit ofthespectrometer.Electrons were incident on thesampleatanangleof30'fromthesamplenormal,andthetake-offanglefor x-rayswas50'.Thespectrawere correctedforthe nonuniformdetectorresponseandastigmaticde-formationin thenondispersiondirection.Completespec-tra wereassembled fromsegmentstaken at successive po-sitionsalongthe Rowland circle. Eachsegmentcoversroughly15A in the siliconLemissionrange.Brems-strahlungbackgroundinducedin electron excitation isfitted as astraightlineand subtracted from thespectra.Forthe3-keVelectronexcitation, weestimate
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